1. Field of the Invention
The invention is directed to improved conductive adhesives for solder-free interconnections in microelectronic assembly processes for attachment of electronic components to a substrate and, in particular, for chip carrier-to-substrate interconnections. These adhesives are characterized by low tensile modulus, low resistivity, high adhesion strength, and durability of these properties during reliability stress conditions of thermal shock, thermal aging, and temperature/humidity (85xc2x0 C./85%RH) exposure of the assembled devices.
2. Description of Related Art
Improved conductive adhesives are required for Pb solder-free interconnection solution as lead/tin (Pb/Sn) alternatives in the fabrication of electronic packaging structures as for single chip and multi-chip electronic modules, typically used in high speed computers, automotive electronics, medical and telecommunication devices, cellular phones, and other consumer products. For reliable product performance with conductive adhesives as interconnecting materials for attaching a ceramic chip carrier or surface mount components as electrical resistor or capacitor, to flexible or rigid printed circuit boards (PCB), it is important for a conductive adhesive as the bonding material to have the requisite properties to withstand and absorb the thermal coefficient of expansion (TCE) mismatch induced material stresses between dissimilar contacting materials. For example, in the case of ball grid array packages (BGA) with ceramic chip carrier-to-board connections where the organic board which is typically the epoxy-glass organic board, FR-4, and the bismaleimide triazine (BT) resin based material has a TCE about 20-35 ppm/xc2x0, and that of a ceramic chip carrier 3-7 ppm/xc2x0, the metal filled organic conductive adhesive as the interconnecting material must be capable of maintaining bond integrity to assure performance reliability of the electronic module during reliability assessment stress exposures involving thermal shock and 85xc2x0 C./85%RH temperature-humidity excursions and in long term operation. Some of the desirable properties of the organic based conductive adhesives for Pb-free interconnections to provide the benefit of Pb elimination in electronic devices include: low stress, low tensile modulus, low resistivity, stable joint resistance with temperature and T/H exposure, high bond strength, void-free bonding, and bond integrity during environmental stress conditions.
The commonly available thermosetting conductive epoxies which are typically derived from glycidoxy ethers of bis-phenol A and bis-phenol F epoxy resins are generally high modulus materials and thus present a concern about the long term functional reliability of the product when these adhesives are used for bonding dissimilar materials with significant TCE differential, for example, ceramic chip carrier to organic printed circuit board (PCB). Also, most of the commercially available formulations of isotropic conductive adhesives are Ag-filled where the issue of silver migration during temperature-humidity exposure is one of the concerns in their use as interconnection materials in place of Pb/Sn solder. The available flexible epoxies are not satisfactory due to the low thermal stability, performance variability, and in some cases have too short a pot life for practical use in a manufacturing environment. Available electrically conductive thermoplastic polymer based compositions are generally solvent-based which have the problem of dispersion stability, resin bleed, voids in the bond line, and performance variability. Electrically conductive silicone elastomeric compositions are yet another class of conductive adhesives for a variety of applications including Pb-free interconnections, elastomeric connectors in electronic module assembly, EMI shielding and heat sink attachment. Although these materials have the compliance and stress absorbing property to qualify for interconnections between materials with significant difference in TCE, these have been found to have limitations in terms of adhesion to Au and other relevant surfaces, problem of bond failure and resistance increase when the assembled structures are exposed to T/H environment.
U.S. Pat. No. 5,699,228 (Alkov et al.), is concerned with a method of interconnecting a leadless multichip module to a printed wiring board using solder-free interconnects to form the electronic module. U.S. Pat. No. 5,528,466 (Lim et al.) is concerned with mounting and cooling a plurality of integrated circuits using elastomeric connectors and a plurality of electrical components having terminals onto a PCB surface. U.S. Pat. No. 5,182,623 (Hynecek) is concerned with an interconnect structure and heat sink.
U.S. Pat. No. 5,700,581, assigned to the assignee of the present invention, is concerned with solvent-free epoxy-siloxane based adhesives for attachment of silicon device chip to ceramic substrate in the fabrication of single chip (SCM) and multichip (MCM) electronic modules. The conductive adhesives described in this patent comprise a siloxane precursor in conjunction with anhydride or hydroxy benzophenones as curing agents, cure catalyst/cure accelerator, and silver flake as the conductive filler. These adhesives are described as having high die shear strength which is maintained when the adhesive bonded chip-substrate assembly is exposed to thermal shock involving xe2x88x9265xc2x0 C. to about +150xc2x0 C. thermal excursions, and temperature-humidity cycling (85xc2x0 C./85%RH). In the case of silicon chip (TCE about 3 ppm) attachment to ceramic substrate (TCE about 3-7 ppm) application, the conductive adhesives are not exposed to thermal expansion mismatch stresses which would occur with the bonded materials that have significantly different TCE. For example, in the case of the bonding of a ceramic chip carrier (TCE 3-7 ppm) to an organic board (TCE 20-35 ppm), there is significant mismatch in the thermal expansion of the two materials and therefore, there is a need for improved conductive adhesives that would have the necessary performance characteristics for interconnection reliability in applications involving TCE mismatched bonded materials. Also, based on the concerns about silver migration with silver filled conductive adhesives commonly described in the prior art and the ones described in U.S. Pat. No. 5,700,581, there exists a need for improvement in the electrically conductive adhesives as alternative interconnections which do not present the problem of metal migration when the assembled device is subjected to reliability stress conditions and to temperature-humidity environment during normal operation.
The present invention addresses the need for improvements in conductive adhesives, especially the necessary requirement for compliant interconnections for absorption of stresses generated when there is significant TCE mismatch between the bonded materials, and the issue of silver migration with silver filled adhesives. The invention provides isotropically conductive adhesives which have low modulus and thus are capable of absorbing thermal stresses with TCE mismatched bonded materials, which have low resistivity and undergo no significant change in resistivity with thermal cycling and temperature-humidity excursions, which maintain high adhesion strength with Au and ceramic, and which employ Au or Pd surface coated Ag as the preferred filler instead of the commonly used Ag flake, and which provide stable performance during reliability stress exposure and assures performance durability in long term operation of assembled devices.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a conductive adhesive which may be used as interconnecting materials in the manufacture of electronic components, and, to attach electronic components to a substrate, and, in particular, to attach a ceramic chip carrier to a substrate or an organic board such as a printed circuit board (PCB).
It is another object of the present invention to provide a method for making electronic components by the use of a conductive adhesive to connect the components such as a silicon chip, or passive electronic components as electrical resistor or capacitor to a substrate such as a printed circuit board and to the electronic components made by the method.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to conductive adhesives as interconnecting materials for solder replacement in electronic module assembly, which adhesives are capable of providing stable and reliable performance of the electronic devices fabricated using these adhesives for interconnections, for example, between a chip carrier and a board, attachment of surface mount components (SMT) to a printed circuit board (PCB), attachment of heat sink for cooling, and for chip attachment to a chip carrier. The desirable properties of the conductive adhesives for these applications include: low resistivity; stable resistance; high adhesion strength with various contacting materials, particularly, Au, lead-tin solder (Pb/Sn), alumina ceramic, glass ceramic, and silicon; maintaining bond integrity during reliability stress exposures involving thermal cycling, thermal shock, and T/H exposure; void-free bond line; thermal stability at least up to 250xc2x0 C.; paste viscosity suitable for screening or dispensing; and polymer-conductive filler dispersion stability preferably that no change in viscosity occurs for at least 8 hrs. at room temperature.
In a first aspect this invention is concerned with solvent-free isotropically conductive adhesives having improved structural and functional performance characteristics in terms of polymer chain flexibility; thermomechanical properties; resistance to cracking; adhesion to Au, lead-tin solder, and ceramic; low moisture absorption, low resistivity, and TCR stability (thermal coefficient of resistance), and other relevant properties for reliable performance as interconnect materials. These adhesives are comprised of high conductivity noble metal filler as the major component dispersed in an epoxy siloxane/preformed polymeric or oligomeric additive/anhydride curing agent/cure catalyst system as the organic carrier. The conductive adhesive formulation is preferably prepared by blending the liquid epoxy siloxane precursor with the polymeric/oligomeric additive, e.g., an alkylacrylate or methacrylate polymer, and an anhydride curing agent and in which the organic constituents are all mutually soluble to form the desired viscosity binder system. A metal filler is added to the binder system typically in the range of 75-90% (wt. %) to form a paste with a viscosity and rheology properties suitable for screening or for syringe dispensing using autodispense tools. The metal filler can be Ag, Ag/Pd, Ag/Au and mixtures thereof, which is preferably in flake form although powder form can also be used. The highly preferred metal filler according to this invention is Au or Pd surface coated Ag or it can be a blend of Ag and Pd or Ag and Au such that the Pd or Au are present up to about 5-20wt % based on the total weight of the filler in the paste. The metal filler can be further modified by using metal particles of different morphology such as powder and flake combination, particle size distribution of polydisperse powder filler to assure maximum packing density, addition of ultrafine particles as Ag, Pd, Au nanoparticles with varying size, particle size distribution, shape, and morphology.
Broadly stated, the invention provides a curable epoxy-siloxane based solvent-free conductive adhesive composition comprising:
a siloxane containing at least two epoxy-functional groups, or mixtures thereof;
a curing agent;
a curing catalyst;
an organic polymeric or oligomeric additive; and
a conductive filler;
wherein the siloxane, curing agent, curing catalyst and polymer additive are mutually soluble.
In another aspect the invention provides a method for making an electronic component assembly, which requires conductive attachment of separate components comprising the steps of:
applying, for example, by screen printing or syringe dispensing, a conductive adhesive on the bonding pads on one of the components to be bonded, the conductive adhesive comprising:
a siloxane comprising at least two epoxy-functional groups, or mixtures thereof;
a curing agent;
a curing catalyst;
an organic polymeric or oligomeric additive; and
a conductive filler;
wherein the siloxane, curing agent, curing catalyst and polymer additive are mutually soluble;
placing and aligning with the contact pads of the other component to be bonded on the adhesive coated pads of the first component; and
heating or otherwise curing the assembled structure to cure the conductive adhesive to provide the desired interconnections between the components.
The conductive adhesives of this invention have one or more of the following distinguishing properties:
1. Low tensile modulus, in the range 5000-450,000 psi, at room temperature.
2. Low resistivity especially with the preferred Ag/Pd and Ag/Au filler system.
3. Excellent adhesion to ceramic, Pb/Sn solder, and plated Au.
4. Adhesion durability under thermal shock and temperature-humidity stress exposure.
5. TCR stability (thermal coefficient of resistance).
6. Resistance to stress cracking.
7. Interconnection integrity during reliability stress testing.
According to this invention, the improved epoxy-siloxane based conductive adhesive composition is a dispersion of a conductive metal filler in an organic binder system comprising of a liquid epoxy siloxane, a polymer such as an alkyl acrylate or methacrylate polymer additive, an anhydride curing agent, and a commonly used epoxide-anhydride cure catalyst such as a tertiary amine with a proton source. The liquid epoxy siloxane selected is such that it forms a soluble admixture with all other solid or liquid constituents of the binder system forming a clear homogeneous solution which is used for dispersing a conductive metal filler to form the conductive adhesive compositions.
The general term xe2x80x9cdiepoxidexe2x80x9d, xe2x80x9cpolyepoxidexe2x80x9d, xe2x80x9cepoxidexe2x80x9d or xe2x80x9cepoxy resinxe2x80x9d is generally interchangeable and defined as any molecule containing more than one alpha-oxirane group capable of being polymerized by ring-opening reactions of the epoxy group. Exemplary widely used epoxy resins include the diglycidyl ethers of bisphenol A and bisphenol F which are made by reacting epichlorohydrin with bisphenol A or bisphenol F in the presence of a basic catalyst. In the preferred siloxane containing diglycidyl ether epoxides used in the invention, the bisphenol A or bisphenol F group is essentially replaced by the siloxane group. Typical siloxane epoxides useful for the purpose of this invention include commercially available bis(1,3-glycidoxy propyl)tetramethyl disiloxane; bis-(1,5 glycidoxy propyl)hexamethyl trisiloxane and related materials some of which are commercially available. These can be used as the pure epoxy component or in combination with non-siloxane epoxides, for example, 1,4-cyclohexane-dimethanol diglycidyl ether; cycloaliphatic epoxides such as 3,4-cyclohexylmethyl-3,4-epoxycyclohexane carboxylate, or the commonly used diglycidyl ethers of bisphenol A or bisphenol F. Other representative siloxane containing epoxy resins and their method of preparation are shown in U.S. Pat. No. 4,480,009 to Berger and in the article entitled xe2x80x9cPolymers From Siloxane-Containing Epoxiesxe2x80x9d by William J. Patterson and Norman Bilow, Journal of Polymer Science: Part A-1, Vol. 7, Pages 1089-1110 (1969), both references being incorporated herein by reference.
Preferred siloxane containing epoxides include: bis(1,3-glycidoxypropyl) tetraalkyl disiloxane; bis-(1,3-glycidoxybutyl) tetralkyl disiloxane; bis(glycidoxy-propyl or glycidoxy-butyl) hexaalkyl trisiloxane; bis(1-3-glycidoxypropyl)bis(methyl 3,3,3-trifluoropropyl) disiloxane; bis(1,5-glycidoxypropyl) tris(methyl 3,3,3-trifluoropropyl) trisiloxane, and related materials, which are represented by the general formula: 
wherein x=2-4; y=1-5, preferably 1-3; R is an alkyl radical represented by the formula, CnH2n+1 where n=1-4; Rxe2x80x2 is the same as R or a 3,3,3-trifluoropropyl radical represented by the formula xe2x80x94CH2CH2CF3.
The preferred epoxy siloxane precursors are, for example, bis(1,3-glycidoxypropyl)tetramethyl siloxane; bis(1,5-glycidoxypropyl)hexamethyl trisiloxane; bis(1,3-glycidoxypropyl)bis(methyl 3,3,3-trifluoropropyl)disiloxane; bis(1,5-glycidoxypropyl)tris(methyl 3,3,3-trifluoropropyl)trisiloxane, and related materials.
The curing agents for the epoxy siloxane precursors may be any of the known curing agents with the proviso that the curing agent be soluble in the epoxide. The curing agent will be typically selected from the group consisting of amines or anhydrides as conventionally used.
In one embodiment of this invention, the preferred anhydride curing agent is a five-membered anhydride which can be liquid or solid so long as it dissolves in the selected liquid epoxy siloxane by mixing at ambient temperature or by heating to an elevated temperature such as up to 50-70xc2x0C.
These include bis-1,2-cyclohexane dicarboxylic anhydride, also named hexahydrophthalic anhydride (HHPA); 4-methyl-1,2-cyclohexane dicarboxylic anhydride, also named hexahydro-4-methylphthalic anhydride (MeHHPA); cis-4-cyclohexene-1,2-dicarboxylic anhydride or cis-1,2,3,6-tetrahydrophthaic anhydride (cis-THPA); methyl-5-norbomene-2,3-dicarboxylic anhydride; and maleic anhydride. These can be used as a single curing agent or as binary or ternary mixtures in the liquid epoxy siloxane.
Other anhydride curing agents include succinic anhydride (SA), dodecenyl succinic anhydride (DDSA), octenyl succinic anhydride; 2-dodecene 1-yl-succinic anhydride; hexadecenyl succinic anhydride. These can be used as a single curing agent or as binary or ternary mixture in the liquid epoxy siloxane.
In general, the amount of epoxide and curing additives are present in substantially equimolar amount, but the epoxide:curing additive molar ratio may vary from about 3:1 to 1:3, e.g., about 2:1 to 1:1.8 although lower or higher ratios may be employed depending on the reactants and properties desired.
The curing catalyst may be any of the epoxide curing catalysts known in the art and will generally be a tertiary amine in conjunction with a proton source such as nonylphenol, ethylene glycol or propylene glycol for anhydride curing epoxy formulations.
Preferred catalyst according to this invention comprise a tertiary amine with a proton source as the 2,4,6-tris-(dimethylaminomethyl phenol) (DMP) and nonylphenol combination although other standard epoxy curing catalysts can also be employed in the adhesive formulations of these formulations. Exemplary amine catalysts include benzyl dimethylamine; tris-dimethylamino methyl phenol; N,N-dimethyl ethanol amine; N,N,N1N1-tetrakis(2-hydroxypropyl) ethylene diamine and the like, which can be used in conjunction with a proton source such as nonylphenol, ethylene glycol, and mixture thereof.
The preferred polymeric or oligomeric additives are acrylate derived polymers. The acrylate polymer additive used in these adhesive formulations include polyacrylates and polymethacrylates and polyalkyl acrylates and polyalkyl methacrylates such as poly(n-butylacrylate and/or n-butylmethacrylate), poly(n-fluorobutyl methacrylate), and polymethyl methacrylate and mixture thereof. In general, the alkyl group may be about C1-C8, preferably C2-C4. The acrylate polymer is miscible in the adhesive binder formulation.
The polymer additives are preferably selected from the group consisting of poly(methylmethacrylate) having molecular weight in the range about 10,000 to about 75,000, preferably between 15,000 to about 40,000; poly(n-butylmethacrylate), poly(n-fluorobutylmethacrylate) having molecular weight in the range about 40,000 to about 320,000, preferably from about 50,000 to about 200,000; and related polymers so long as these form a miscible blend with the epoxy siloxane-anhydride curing agent system.
The conductive metal filler used in the adhesive compositions described in this invention can be silver (Ag), gold (Au), silver coated with palladium (Ag/Pd), silver coated with gold (Ag/Au), and mixtures thereof. The fillers which are in the form of flake or powder, and the Ag/Pd and the Ag/Au are either surface coated Pd or Au coated Ag, or these can be blends of Ag and Pd or Au in the form of flake or powder such that the Pd or Au are present in an amount in the range of about 5 to 20 wt. % relative to Ag. The particle size of the metal filler varies in the range 2-30 xcexcm or higher. These compositions are available from Degussa Corporation.
A preferred electrically conductive filler is Pd or Au surface coated Ag flake or powder or a blend of Pd and Ag or Au and Ag, having less than about 10 micron average particle size, although up to about 30 microns or higher may be used. Filler amounts up to about 90% or higher by weight of the total formulation may be employed with preferred amounts of 70% to 85% being typically employed. The fillers may be blended into the formulation using conventional techniques known for obtaining polymer/filler dispersions such as high shear mixers or rotary mixers.
These adhesives can be used for attaching electronic components to PCBs, to attach solder balls on contact pads of a substrate, to attach silicon chips to a substrate, and to attach a chip carrier to a PCB. A typical bonding process using the adhesives of this invention is exemplified by attachment of a ceramic chip carrier to a PCB board typically, epoxy-glass FR-4, according to which the conductive adhesive is screened onto the Au surface contact pads on a ceramic ball grid array (BGA) module which is then positioned onto the PCB such that the adhesive regions on the chip carrier are brought into contact with the contact solder pads, which are preferably Au coated solder contact pad on the printed circuit board. Alternatively, in addition to screen printing the conductive adhesive onto the contact pads of the chip carrier it can also be deposited onto the Au coated solder contact pads on the PCB by screen printing or by the syringe dispense technique, bringing in contact and to aligning the chip carrier and PCB contact pads, and heating the resulting assembly up to 160-170xc2x0 C. under N2 for example in order to cure the adhesive as described above. Effect of stress exposures on the structure integrity is evaluated by subjecting the assembly to 0xc2x0 C. to 100xc2x0 C. thermal shock and 85xc2x0 C./85% RH temperature-humidity excursions.